1. Field of the Invention
The present invention relates to an MEMS type acceleration sensor having a weight structure.
2. Description of the Invention
In recent years, attention is focused on a technique of fabricating a significantly fine structure having a size of about a few 100 μm with the use of the micromachining technology to which the semiconductor micromachining technology is applied. For this microstructure, applications to an optical switch and a radio-frequency (RF) component are investigated in various sensors and the field of optical communications. Generally, since these components applied with micromachining are fabricated with the use of the Si process, they can be integrated with a signal processing LSI on a chip. Consequently, a system having a certain function is configured on a chip. Accordingly, the device having this function is called Micro-Electrical-Mechanical-Systems, MEMS, in the Unite States and Micro-System-Technology, MIST, in Europe.
One of the MEMS components widely used is an acceleration sensor. The acceleration sensor is widely applied to a supplemental restraint system for automobiles, a measuring system of information about underground environments such as earthquake activity, and an earthquake-proof system for IT components. For example, JP-A-7-225240 discloses a piezo acceleration sensor among the MEMS components. The structure is that a sensor chip is mounted on a mounting board by an adhesive of a molding resin, in which the sensor chip is formed of a weight, a beam and a frame. Then, the weight is surrounded by the frame. The weight is joined to the frame by a plurality of the beams, and the weight is separated from the board by being supported by the beams. In the acceleration sensor, the weight senses acceleration toward the mounting board in the vertical direction, and the weight moves vertically. The resistance value of the beams supporting the weight is varied at this time, and thus current is carried through the beams to sense the resistance value for measuring the acceleration.
However, the traditional structure has a problem that bonding the sensor chip to the mounting board causes the molding resin to flow into the inside of the frame, the resin reaches under the weight and the weight is bonded to the mounting board.
It is possible to avoid the influence by reducing the thickness of the weight or raising the height of the frame to have space between the weight and the mounting board even though the molding resin flows into the inside of the frame. However, when the distance between the weight and the mounting board is increased too much, a problem arises that the weight greatly moves in the direction of the mounting board and en excessive stress is applied to the beams supporting the weight to destroy the beams in the case where a great acceleration is applied.